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Symplectic Geometry Symplectic Geometry Nicholas Proudfoot Department of Mathematics, University of Oregon, Eugene, OR 97403 These notes are written for a ten week graduate class on symplectic geometry. Most of the material here is included in Mich`eleAudin's book Torus actions on symplectic manifolds, which I used heavily in preparing these notes. There are a few topics that we cover that are not in Audin's book, such as polygon spaces, cohomology of flag manifolds, and CS/GKM theory. More significant, however, are the omissions; Audin's book contains too much material for a course of this length, and I have included just enough to accomplish my main goals, which are as follows: • Introduce interesting classes of examples of symplectic manifolds, including partial flag man- ifolds, polygon spaces, and toric varieties. • Prove Delzant's theorem, which says that the constructive definition of a toric variety (start with a polyhedron and build a space out of it) is equivalent to the abstract definition. • Use Hamiltonian torus actions to understand Betti numbers (via Morse theory) and coho- mology rings (via various equivariant tricks) of symplectic manifolds. I have also been strongly influenced by Ana Cannas da Silva's Lectures on symplectic geometry [CdS] and by conversations with Allen Knutson, from whom I learned the subject. What is all of this about? A symplectic form on a manifold X is a closed, nondegenerate 2-form. A nondegenerate 2-form is a smoothly varying perfect pairing on the tangent spaces of X that is skew-symmetric (I'll remind 2 3 you what \closed" means in a little while). For example, let X := S = fp 2 R j jpj = 1g: Then 3 for all p 2 X, TpX = fq 2 R j p · q = 0g: The 2-form ! given by the formula !p(q; r) = p · (q × r) is symplectic. (Since X is a 2-manifold, all 2-forms are automatically closed.) If you are looking down at the sphere at the point p, this means that any unit tangent vector at p pairs to 1 with the vector obtained by rotating the original vector 90 degrees counter-clockwise, but it pairs trivially with itself (this is always the case, by skew-symmetry). It is interesting to compare this symplectic form to the usual Riemannian metric. A Riemannian metric is a smoothly varying perfect pairing on the tangent spaces of X that is symmetric. In the case of the standard Riemannian metric on S2, any unit tangent vector pairs to 1 with itself, but pairs trivially with vectors that differ from it by a 90 degree rotation. One interesting thing that one can do with a Riemannian metric is define the gradient of a function. Given a smooth function f : X ! R, its derivative df is a 1-form. That means that, for any p 2 X, we obtain a linear function on TpX taking a tangent vector v to dfp(v), the directional 1 derivative of f at p along v. This linear function must be given by pairing with some vector, and that vector is called the gradient of f at p. For example, if f is the height function on S2, then its gradient always points in the direction of the north pole. At the equator it has norm 1, and it shrinks as the latitude gets higher or lower, vanishing at the two poles. One can also define the symplectic gradient of f, using the symplectic form rather than the Riemannian metric. That is, the symplectic gradient vf is characterized by the condition that !p(vf (p); v) = dfp(v) for all v 2 TpX. (It was important for me to write it out to specify the order, which is something I didn't have to worry about in the Riemannian case!) The symplectic gradient is obtained from the Riemannian gradient by rotating 90 degrees clockwise. When you have a vector field on X, you can consider the family of diffeomorphisms of X given by flowing along that vector field. In our example, flowing along the Riemannian gradient may be described as \reverse oozing". On the other hand, flowing along the symplectic gradient is spinning (counter-clockwise if you are looking down at the north pole). This is a nicer flow in many respects. For example, it preserves level sets of the function, and it does not \deform" the sphere. These two phenomena are general facts about symplectic gradients, as we shall later see. Another nice property is that the flow is periodic; this does not hold in general, but it happens a lot, and we will be particularly interested in functions whose symplectic gradients induce periodic flows. Such functions are called U(1) moment maps. One of the main goals of the class will be to use U(1) moment maps to study the cohomology of X. For example, we will prove that a U(1) moment map is always a \perfect Morse-Bott function", which means that the Betti numbers of X can be read off from the critical sets of the function along with the behavior of the flow near those critical points. (In the case of S2, the south pole contributes to H0(S2) because the flow is clockwise around the south pole, while the north pole contributes to H2(S2) because the flow is counter-clockwise.) We will then use equivariant cohomology to show that products of cohomology classes can also be computed in terms of local data at the fixed points, and use this to understand the cohomology ring of X. The previous two paragraphs are about U(1) symmetries of symplectic manifolds, but there is no need to stop there. The more symmetries that we have, the more we can say about the topology of our manifold. Throughout the text, the two main sets of examples on which we will focus are flag manifolds and toric varieties. These are, in two different senses, the \most symmetric" symplectic manifolds: a flag manifolds is a homogeneous space for the unitary group U(n), while toric varieties have the largest possible abelian symmetry group for their dimension. These two classes of symplectic manifolds also provide strong connections to other fields of mathematics. The geometry of the flag manifold is central to the study of representation theory of U(n), while toric varieties interact richly with with the combinatorics of polytopes. We will see how Stanely used toric varieties to prove half of the McMullen conjecture on f-vectors of polytopes. If we have time, we will also use the Kirwan-Ness theorem to translate a problem about tensor product multiplicities of U(n) representations into symplectic geometry. 2 Contents 1 Differential topology 4 1.1 Manifolds . .4 1.2 Differential forms . .4 1.3 Lie groups and Lie algebras . .6 1.4 Lie group actions on vector spaces . .8 1.5 Lie group actions on manifolds . .9 2 Symplectic manifolds 12 2.1 Definition and basic properties . 12 2.2 Examples . 14 2.3 Darboux's theorem . 18 3 Hamiltonian actions 20 3.1 Hamiltonian vector fields . 20 3.2 Moment maps . 20 3.3 Symplectic reduction . 25 3.4 Toric varieties . 28 4 Morse theory 34 4.1 Morse theory oversimplified . 34 4.2 Poincar´epolynomials of symplectic manifolds . 38 4.3 Morse-Bott theory . 41 4.4 The Atiyah-Guillemin-Sternberg theorem . 43 4.5 Proof of Delzant's theorem . 45 5 Equivariant cohomology 49 5.1 The Borel space . 49 5.2 Thinking about (equivariant) cohomology classes . 50 5.3 Torus equivariant cohomology of a point . 53 5.4 Other groups . 54 5.5 The equivariant cohomology of the 2-sphere . 55 5.6 Equivariant formality . 57 5.7 (Equivariant) cohomology of toric varieties . 58 5.8 The localization theorem . 60 5.9 The localization formula . 63 5.10 CS/GKM theory . 67 5.11 Cohomology of polygon spaces . 69 3 1 Differential topology We'll begin with a quick review of some basic definitions in differential topology and Lie theory. 1.1 Manifolds I'll start by listing a few things that I will not review, because you know them so well already. • The definition of a manifold and of a smooth map between two manifolds. • If X is a manifold and p 2 X, then we have a real vector space TpX, called the tangent space to X at p. • If X is a manifold, then we have a manifold TX, equipped with a smooth map TX ! X, such that the preimage of p is canonically identified with TpX. The manifold TX is called the tangent bundle of X. • If f : X ! Y is a smooth map and p 2 X, there is an induced linear map dfp : TpX ! Tf(p)Y . These linear maps fit together into a smooth map df : TX ! TY . • If f : X ! Y is a smooth map, a point q 2 Y is called a regular value if dfp : TpX ! TqY −1 −1 −1 is surjective for all p 2 f (q). In this case, f (q) is a manifold, and Tpf (q) = ker dfp. 1.2 Differential forms Let X be a manifold. Definition 1.1. A vector field on X is a smooth section of the map TX ! X. Intuitively, this means that it is a smoothly varying choice of tangent vector at every point. Thus, if v is a vector field on X and p 2 X, then v(p) 2 TpX ⊂ TX. We will sometimes write vp instead of v(p). We will denote the set of vector fields on X by VF(X).
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